1. Field of the Invention
This inevntion relates to an automatic control device operative to adjust the clearance in a seal of the labyrinth type of a turbo-machine.
2. Discussion of the Background
Fluid-tightness between fixed and rotary parts of turbo machines frequently make use of seals of the labyrinth type comprising, on the one hand, on the rotary part, members in the form of small tips, having a number varying in dependence upon the operational conditions and in accordance with various technological parameters, and, on the other hand, on the fixed part lying opposite thereto, a member forming a wear and fluid-tight seal termed an "abradable", that is to say wearable as a result of friction during possible contact with a tip without giving rise to appreciable damage to the latter, this wear seal member being carried by an annular carrier connected to a fixed structure of the turbo machine. Such labyrinth seals can be disposed, for example, between various rotary stages of a compressor or of a turbine, and fixed parts (or rotary at a different speed) adjacent thereto. The tips are in these cases supported by distance pices or rings and the wear seal member is secured on the stator (or in the rotary part preferably having the lower speed).
In another specific use of the labyrinth seals which provides a more direct application envisaged by the invention, these seals are disposed between various enclosures of the turbo-machine and they are to be found particularly at the end of an outer enclosure of the combustion chamber casing, on the one hand, the side of the outlet of the compressor and, on the other hand, the side at the inlet of the turbine. In this case the correct fluid-tight operation of the seal is more complex. In practice a pressure balance between the various enclosures of the turbo-machine is normally sought. A controlled flow of air is also desired in the enclosures with a view to creating particular air currents for cooling eventually usable in other zones of the turbo machine and thus it may be desirable to control with the highest precision the air flows termed "loss flows" traversing this type of labyrinth seal and of which the control reflects on the various results such as the efficiencies of the turbo machine or the useful life of the various parts. Apart from various operational conditions, such as the pressures in the enclosed spaces, one of the fundamental parameters on which this air flow control depends is the clearance during operation between the upper parts of the tips and the wear seal member.
Various proposals for overcoming the problems which have been posed have been put forward and in particular for maintining a controlled value for the clearance between the tips and the wear seal member in a labyrinth seal, such that they will be maintained under operational conditions of the turbo-machine, both at a stabilized rating or during transitory rating phases. Thus, in FR-A-2 437 544 in the name of the present Applicant a labyrinth seal is described in which the carrier of the wear seal member is surrounded by an annular duct connected, at its downstream end, to an air supply provided in the wall of the combustion chamber casing whilst its other end discharges upstream of the labyrinth seal member into space at a lower air pressure surrounding the compressor shaft. The control of the amount of air flow for cooling the seal makes use in this case of an adjustable discharge vane operative on the basis of an operational parameter of the turbo machine. This mode of control has nevertheless various disadvantages inherent in the method because it relies, on the one hand, on a derivative of a complex chain of control thus multiplying the risks of an accident or defective operation arising from the vanes and other accessories and, on the other hand, the response times, particularly during the transitory phase ratings, thus risking too great delay for ensuring totally satisfactory operation.
Another known device according to FR-A-2 449 789 seeks to achieve cooling of a labyrinth seal disposed downstream of a turbo machine compressor and provided with this aim with passages traversing the stator of the seal through which is supplied cooling air delivered between two upstream teeth of the rotor of the seal.
In accordance with GB-A-1 525 746, labyrinth seals are disposed successively from upstream in the downstream direction in the three zones disposed radially-inwardly of the downstream part of a combustion chamber and radially-inwardly of the inlet guide vane array of the associated turbines. In order to reduce the overall losses produced by the labyringth seals, the device described collects the losses derived from the first two labyrinth seals and reintroduces this air into the third labyrinth seal between the teeth of the rotor of the seal derived from apertures traversing the stator of this seal.